Metastasis remains a significant clinical challenge and a major cause of cancer-related death. The mechanisms that govern metastasis are complex, and extend beyond genetic or epigenetic alterations in the tumor cells themselves. It is now well-recognized that host-derived elements, namely macrophages that either dwell within or are recruited to the metastatic tumor microenvironment (TME) greatly influence outcome. Ordinarily, macrophages are critical for host defense, including cancer. However, during disease progression and under the influence of tumor-associated factors (TAFs), macrophages may transition from ?antitumor defenders? to ?pro-tumor remodelers? where they now exert pro-metastatic activities. The precise molecular events that determine such functional states remain unresolved. Understanding the molecular bases by which macrophage ?defender? activities are subverted by TAFs has potential implications for therapies aimed to reinstate their tumoricidal abilities. While it is thought that macrophage functional diversity is governed by transcriptional pathways, transcriptional regulators that support macrophage antitumor activity within the metastatic TME remain to be fully understood. Our efforts to address this gap in knowledge have unveiled the importance of interferon regulatory factor 8 (IRF8) for driving or sustaining macrophage anti-metastatic activity. Thus, we posit that IRF8 is an integral transcriptional positive regulator of an ?antitumor defender? macrophage phenotype. Utilizing a conditional knockout model of IRF8 deletion, our preliminary data demonstrate that mice with IRF8-deficient macrophages have significantly more metastasis compared to the wild-type controls. Since our preliminary data were generated using a genetic model, it remains unknown when and how IRF8 is downregulated in macrophages during tumor progression in vivo. In this proposal, we hypothesize that macrophage IRF8 expression is downregulated by certain TAFs, including G-CSF, IL-10, or TGF-? within the metastatic TME. We seek to determine the dynamics and mechanisms of IRF8 downregulation and to identify IRF8-mediated mechanisms of macrophage anti-metastatic activity. To begin to dissect the mechanism by which IRF8 regulates the macrophage response, we conducted RNA-seq studies and identified significantly reduced chemokine ligand 7 (CCL7) expression by IRF8-deficient macrophages. Interestingly, CCL7 has been identified as a potent chemoattractant for myeloid cell and T cell recruitment, which can mediate pro- or antitumor effects. Such divergent functional activities are likely context-dependent and, in this proposal, we hypothesize that IRF8 promotes macrophage anti-metastatic activity through a novel antitumor CCL7-CD8+ T cell dependent mechanism. Subsequent studies will focus on anti-TAF strategies to recover macrophage IRF8 expression to improve anti-metastatic responses either alone or combined with immune checkpoint blockade. Overall, our studies will provide new insights into macrophage-tumor biology, with potential implications of macrophage IRF8 expression levels for therapeutic targeting in cancer.